PURPOSE: To improve the high-frequency characteristic of a laser diode driving circuit and to enable the high-speed operation thereof by a construction wherein the base of one of transistors constituting a differential amplifier circuit and the base of a transistor constituting a constant current circuit are connected together through the intermediary of a capacitor.

CONSTITUTION: A capacitor 21 is connected between the base of a transistor 16 of a differential amplifier circuit and the base of a transistor 18 of a constant current circuit. When it is assumed that a signal impressed on an input terminal 1 changes from H to L, for instance, an emitter current of a transistor 15 starts to move in the direction of increase and the emitter current of the transistor 16 in the direction of decrease, while a current (ie) of the constant current circuit decreases in the shape of a short pulse, so as to accelerate the decrease of the emitter current of the transistor 16. Since the decrease of the emitter current is equivalent to the decrease of a collector current of the same transistor, a current flowing through a laser diode 14 decreases sharply, and consequently the laser diode 14 stops emission of light instantaneously.